Scattered radiation is basically caused by the interaction between the object of interest and primary radiation emanating from the focus of a radiation source. Because of this interaction, it is incident on a radiation converter of a radiation detector from a different spatial direction from that of the primary radiation and causes artifacts in the reconstructed image.
To reduce the detected scatter component in the detector signals, the radiation converters are therefore preceded by collimators. Such collimators have absorber elements whose surfaces are aligned radially to the focus of a radiation source in a fan-like manner so that only radiation from a spatial direction in line with the focus can be incident on the radiation detector.
Even a slight tilt or incorrect positioning of the collimator relative to a radiation converter can cause shadowing of the active regions of the radiation converter, resulting in distortion, i.e. a reduction in the achievable signal-to-noise ratio. A particular challenge for designing a radiation detector is therefore to produce a collimator of very high mechanical strength so that positioning accuracies to within a few μm can be maintained.
These stability requirements are particularly important when the collimator is used in a CT scanner, due to the centrifugal forces acting on the collimators during rotation. In addition, the radiation detectors increasingly have a higher z-coverage in order to enlarge the scan field of view. This increases the width to be spanned by the collimators in the z-direction, thereby increasing the risk of collimator instability.
Due to the enlargement of the radiation detector in the z-direction and in the case of dual-source systems in which two source/detector systems disposed in one scanning plane and offset by a fixed angle in the φ-direction are operated simultaneously to obtain projections, not only scatter suppression along the φ-direction is required but also collimation in the z-direction. Collimators which suppress scatter in one spatial direction only, usually in the φ-direction, are termed one-dimensional (1D) collimators. Collimators producing a collimating effect in two spatial directions are accordingly known as two-dimensional (2D) collimators.
To meet the stability requirements for a 1D collimator, in the known case as described in the publication DE 10 2007 051 306 A1, absorber elements aligned along a z-direction are segmented and mounted in a housing. Segmentation of the absorber elements is performed with the aim of reducing the manufacturing costs while at the same time meeting tighter engineering tolerances. The mechanical stability of the 1D collimator is provided by using a housing in which the plate-shaped absorber elements are precisely aligned and mounted. As a supporting structure, the housing comprises two bridge-like frame sections which are mechanically fixed by a plug-in connection. Housing shapes are also disclosed wherein the frame sections run alongside the absorber elements in each case.
However, the disadvantage of both types of housing is that the frame sections are in the beam path of X-ray radiation to be detected. Due to the nature of their material, the frame sections cannot be completely transparent to X-ray radiation, which means that providing mechanical stability via the housing involves unwanted attenuation of the X-ray radiation and additional scatter generation. This disadvantage is particularly apparent in the case of bridge-shaped housings where the edges of the absorber elements are spanned by the frame sections in one plane. Circumferential frame sections also have the disadvantage that the absorber elements can only be lined up with pitch discontinuities because of an intervening wall.
A 2D collimator is described, for example, in DE 10 2005 044 650 A1. It has a two-dimensional structure with cellular radiation channels. In the disclosed case, the lamellar absorber elements are interconnected cruciformly in a form-fit manner by corresponding slits in the absorber elements to be connected. 2D collimators are also known which are produced by laser sintering of radiation-absorbing metal powder or by stacking a plurality of cast or injection-molded individual gratings made of tungsten-powder-filled polymers. The 2D collimators are also segmented into individual 2D collimator modules to reduce the manufacturing cost/complexity and narrow the manufacturing tolerances, the segment size usually corresponding to the segment size of the radiation converter's detector tile mounted in a detector module. To construct the 2D collimator and produce a mechanically stable arrangement of the 2D collimator modules, these are glued directly to the respective detector tiles.
However, in the event of a defect, glued-on 2D collimator modules cause warping both of the 2D collimator module and of the detector tiles, as nondestructive removal is generally no longer possible. In addition, the detector tiles are subjected to corresponding centrifugal forces by the glued-on 2D collimator modules during rotation.